1. Field of Invention
The present invention is related to a surface treating method for enhancing wear resistance by means of depositing a sulfurizing layer of 5 .mu.m or less thickness, in which iron sulfide and oxide are mixed, on carbon steels for constructional use, alloy steels, special steels and iron-based forgings and castings and, more particularly, hardening heat-treated iron-based sliding parts, such as by carburizing, nitriding and induction heating. The present invention is furthermore related to iron-based material, on which the wear-resistant layer is applied.
2. Description of Related Art
The sulfur-permeation method, which is usually referred to as the sulfurizing treatment, was developed in France 1953. The following facts are known in this treatment: Active sulfur (S.sup.0) is diffused into the iron surface of a workpiece and a sulfurizing layer consisting of such iron sulfides as FeS and FeS.sub.1-x, and oxides such as FeO and Fe.sub.3 O.sub.4 are formed; when the sulfurizing layer is brought into sliding contact with the opposite material, the coefficient of friction decreases, so that adhesion and fusion become difficult to occur and hence the wear resistance is improved.
The following methods are representative among the variously devised sulfurizing methods.
(1) Solid method or pack method. Heating is carried out in the solid FeS and graphite.
(2) Immersion method. A workpiece is immersed in a sulfur-containing aqueous solution or molten salt and is then heated to 300.degree. C. or higher.
(3) Molten salt method. A workpiece is immersed, at 500.degree. C. or higher, in neutral sodium-chloride containing salt, to which sulfur is added. Alternatively, a workpiece is immersed, at 500.degree. C. or higher, in cyanic acid-containing salt, to which sulfur is added.
(4) Gas method. Heating is carried out in mixed gas atmosphere of hydrogen sulfide with hydrogen or ammonia at 500.degree. C. or higher.
(5) Electrolytic method. Molten salt of thiocyanic acid is electrolyzed at 170.degree. C. or higher.
Wear resistance of the sulfurizing layer varies depending upon various factors, such as its surface hardness and roughness, kind of material on which the sulfurizing layer is deposited, and wearing conditions, e.g., surface pressure, contacting mode and kind of lubricant. It is, therefore, very difficult to attain stable wear resistance of the sulfurizing layer, in view of the fact that the above factors greatly vary. Since surface hardness of the sulfurizing layer is considerably lower than that of the surface hardening, such as carburizing and nitriding, the sulfurizing layer applied on a low-hardness material is greatly mechanically damaged and is hence not advantageous.
Accordingly, developments have been directed toward simultaneous or consecutive sulfurizing and nitriding methods, such that a hard surface layer is first formed by nitriding and then a wear-resistant layer, i.e., a sulfurizing layer, is formed. The recently industrialized methods are reducing molten salt method based on the nitriding and modified to enable sulfurizing, the gas method and the ionic method.
In the reducing molten salt method, 10% or less of a sulfur compound such as sodium thiosulfate or sodium sulfide is added to the cyanic acid and nitriding cyanic compound. The so prepared mixture is melted and is used for sulfurizing and nitriding. The treatment temperature is as high as 500.degree. C. or higher and the treatment time is as long as 2 hours or more. Therefore, when a high level of the sulfide content in the salt bath is selected, a large amount of iron sulfide is permeated on the workpiece, while the sulfurizing layer is liable to separate from the surface of the workpiece. In the surface region, where the sulfur concentration is high, the permeation and diffusion of nitride is so retarded that only a thin nitride layer can be formed. In order to eliminate such problems, the main trend of the reducing molten salt method is directed to a low-sulfur type with the sulfur-compound level suppressed to as low as a few percent (Japanese Examined Patent Publication (kokoku) No. 59-6911). This method can also meet the requirements to enhance the dimensional accuracy of the sulfurized parts.
When implementing the reducing molten salt method, the sulfur of the molten salt bath must be analyzed to maintain the level of sulfur and to protect the molten bath from oxidation. In addition, when the treated workpiece is washed, the cyanic acid may decompose to yield a free cyano compound. This must be subjected to waste-liquor treatment in the light of pollution control.
The gas method resides in sulfurizing and nitriding in the gas mixture environment of nitrogen, ammonia and hydrogen sulfide. The treatment temperature is as high as 500.degree. C. or higher and the treatment time is as long as 3 hours or more. This method is flexible, since the gas composition can be optionally adjusted to control the thickness of the sulfurizing and nitrided layers. However, the hydrogen sulfide gas having high specific gravity and the ammonia gas must be uniformly mixed. The gas pressure must be controlled. The furnace must be constructed so as to maintain uniform gas composition therein. These are important factors in implementing the gas method. Furthermore, equipment plant for recovering the hydrogen sulfide gas from the waste gas must be installed, which increases the cost of the plant.
The ionic method resides in sulfurizing and nitriding by glow-discharging a gas mixture of ammonia and hydrogen sulfide under reduced pressure. The treatment time is as long as four hours or more. This method is flexible, as well, since the gas composition can be optionally adjusted to control the thickness of the sulfurizing and nitrided layers. However, this method involves similar difficulties and cost problem as in the gas method.
When the above described sulfurizing and nitriding methods are compared, all the methods share the common points that the total thickness of the sulfurized and nitrided layer is as thick as 10 .mu.m or more thick, and further peaks of iron sulfide, such as FeS and Fe.sub.1-x S, are detected by the X-ray diffraction. Quantitatively speaking, the ratio of X-ray diffraction peak intensity indicates that iron sulfide in the gas and ionic methods is greater than that in the reducing molten iron method. However, how much the proportion of sulfide relative to the nitride or iron oxide in the sulfurizing layer is not clear. Notwithstanding this unclarity, it is reported that the wear resistance has a relationship with the amount of iron sulfide on the specimen surface, with regard to the test specimens prepared using various kinds of materials and by changing the surface roughness.
The Corvet method, which was developed in 1964 and industrialized in 1970 in France, has a noteworthy feature that even the hardening heat-treated material such as carburized or nitrided material can be sulfurized. Its patent is entitled "Method for Sulfurizing by Anodic Electrolysis of Molten Salt" (Japanese Examined Patent Publication (kokoku) No. 63-12158). It discloses an electrolytic condition: temperature--170.degree. C. or higher; current density--1.5-4 A/dm.sup.2 ; and, treatment time--4-20 minutes. A layer of iron sulfide (FeS, FeS.sub.2) and iron oxide (FeO, Fe.sub.3 O.sub.4) is deposited on the anode under the reaction of Fe.sup.2+ dissolved from the anode with S.sup.2- yielded by the decomposition of thiocyanic acid. It would be more pertinent based on such disclosure to refer to the Corvet method as the sulfide depositing method rather than the sulfur permeation method. Since the iron dissolves from the anode, its surface is disadvantageously roughened. The iron oxide and sulfide suspend and gradually accumulate in the bath. Under such circumstance of suspension and accumulation, the proportion of iron oxide to the iron sulfide in the sulfurizing layer varies. This necessitates installation of a facility for removing the suspended materials from the bath and also to control the content of suspended materials in the bath. A pollution problem arises due to the free cyano compound as in the reducing molten salt method.
The sulfurizing layer formed by the Corvet method is from 5 to 10 .mu.m thick and hence is thick. In the present specification, FeS and Fe.sub.1-x S formed by the Corvet method is not identified by the X-ray analysis method. The sulfurizing layer is allegedlly composed of FeS, FeS.sub.2 and FeO. Also, the wear resistance decreases with the increase in FeS content to 30% or less.
The present inventors carried out fluorescent X-ray analysis of the above described sulfurizing and nitriding method and the electrolysis method by using a tester mentioned in the Example, for the purpose of investigating the quantity of iron sulfide in the sulfurizing layer. The result of sulfur (S)-count measurement is shown in Table 1.
TABLE 1 ______________________________________ Sulfurizing and Sulfurizing Nitriding Methods Method Molten Salt Gas Ionic Electrolysis Method Method Method Method ______________________________________ S Count 1 or 5 or 5 or 17 or less Number less less less (kcps) ______________________________________
Since the range of thickness of a layer capable of analysis by fluorescent X ray is from approximately 0 to 10 .mu.m, this analysis is not highly accurate for measuring the S count number of a sulfurizing layer of 10 .mu.m or more thickness. This analysis is, however, a relatively simple and easy method for indirectly determining the quantity of iron sulfide in the above sulfurizing layers. As a result of the analysis, it turned out that the amount of iron sulfide in terms of the S count is approximately 20 kcps or less, which is said to be considerably low.
The sulfurizing and nitriding methods and the sulfurizing method of the carburized or nitrided parts are broadly applied for the surface treatment of wear-resistant sliding parts made of general constructional steels, tool steels and the like. The parts treated by these methods are resistant against seizure, scoring wear and pitching wear under high pressure. The sulfurizing layer formed by any one of these methods has a thickness of more than 5 .mu.m. This involves a problem, when applied to precision parts, because the dimensional accuracy is lessened.
The amount of iron sulfide in the sulfurizing layer formed by the sulfurizing and nitriding methods is as small as one third or less of that of the electrolytic sulfurizing method of the surface hardened material.
The surface roughness of the nitrided and then sulfurized material and surface-hardened and then electrolytically sulfurized material reflects that of the material prior to the sulfurization. The latter surface roughness, in turn, reflects that of material prior to the nitriding or surface hardening. Therefore, it is not yet clarified in the prior art to what extent the iron sulfide in the sulfurizing layer contributes to the wear resistance.